Many air conditioning systems used in larger commercial buildings use control of airflow in various ducts as a means for controlling the air conditioning, i.e. heating, cooling or ventilation, in various rooms or spaces of the building. One desirable way of controlling airflow is through the use of fans or blowers having a variable flow rate. Some such fans use controllable dampers or vanes either on the outlet of the blower or on the air inlet thereto, as a means for controlling the airflow volume of the blower. However, such techniques are relatively inefficient in that the damper or other control device causes a considerable loss of energy. A preferable technique is to control the speed of the blower itself, and a number of prior art techniques have been proposed for that purpose.
Variable pulley drive systems have been used for variable speed drives to blowers. In such systems an electric motor drives a fan or blower through a variable pulley system. Such variable pulley systems generally have a variable pulley or sheave and a fixed pulley or sheave with a V-belt coupling them. The variable sheave has one or both of its faces moveable so that the drive belt may ride lower or higher in the pulley depending on the spacing between the movable faces, thus changing the effective pitch diameter of the pulley and the drive ratio system. Control of the drive ratio is accomplished by an actuator which varies the distance between the pulleys which, in conjunction with the spring loaded variable pulley, controls the depth that the V-belt rides in the pulley and thus its driving ratio.
One type of prior art variable speed drive as shown in U.S. Pat. No. 4,378,199 uses a variable sheave on a motor shaft coupled to a fixed sheave which in turn is coupled to drive the fan. The motor is mounted on a platform which slides toward and away from the fixed sheave, under control of a worm gear or other actuator. Although such systems do work to control drive ratio and hence speed of the fan, they are subject to certain disadvantages in that the horizontal slide for the motor can bind or stick, resulting in faulty operation. A typical large drive motor may weigh some 300 pounds. In addition, it is not uncommon that there would be a 100-pound force applied through the drive belt, which is off center with respect to the slidable frame, and these factors can lead to binding of the sliding carriage.
Another type of prior art device as shown in U.S. Pat. No. 4,381,174, uses a drive motor with a variable sheave on the motor, wherein the motor is mounted on a frame that can pivot the motor through an arc in a manner that varies the distance between the pulleys. Such a pivoting technique overcomes the problems of binding in the sliding system discussed above, but it too is subject to certain disadvantages. Because alignment of the two pulleys is critical in order to avoid excessive wear of the drive belt, such prior art designs should use a two-sided movable sheave pulley, i.e. one in which both sides move inwardly or outwardly as the case may be, to change the effective drive diameter without affecting the lateral position and hence alignment of the drive belt. However, if one side of the movable sheave pulley sticks or does not move as freely as the other, upon the changing of the distance between the pulleys to change drive ratio, there can be a net lateral shifting of the drive belt at the motor end. Since these drive belts are engaged firmly along their sides, and since the drive force is considerable, even a slight altering of alignment drastically increase the wear on the belt which can cause premature failure. This in turn increases cost, not only for the maintenance work required but the considerable cost of the belt itself.